Method of improving stability of boron hardenability effect in alloy steels

ABSTRACT

The method of stabilizing the hardenability effect in quench hardenable hypoeutectoid steel containing nitrogen as influenced by time and temperature is decreased by aluminum, columbium and vanadium and increased by titanium and zirconium.

United States Patent 1- Melloy et a1.

[54] METHOD OF IMPROVING STABILITY OF BORON HARDENABILITY EFFECT IN ALLOY STEELS [75] Inventors: George F. Melloy, Bethlehem, Pa.;

John C. Russ, Woburn, Mass.

[73] Assignee: Bethlehem Steel Corporation [22] Filed: Nov. 4, 1970 [21] Appl. No.: 86,962

Related U.S. Application Data [63] Continuation-impart of Ser. No. 824,749, May 8, 1969, abandoned, which is a continuation of Ser. No. 541,210, April 8, 1966, abandoned.

[52] U.S. C1. ..l48/2 [51] Int. Cl. ..C21d 7/14 [58] Field of Search ..75/123, 123 B, 124, 129, 58;

[56] References Cited UNITED STATES PATENTS 2,280,283 4/1942 Crafts ..75/123 B 1 Feb. 20, 1973 Journal of Research of NTL. Bureau of Standards, Res. Paper RH 1815, Vol. 39, July 1947, Digges et 211., pgs. 67-80, 82, 83, 85, 86, 90, 92, 93, 98 & 106.

Primary ExaminerChar1es N. Lovell Attorney.loseph J. OKeefe [57] ABSTRACT The method of stabilizing the hardenability effect in quench hardenable hypoeutectoid steel containing nitrogen as influenced by time and temperature is decreased by aluminum, columbium and vanadium and increased by titanium and zirconium.

1 Claim, 7 Drawing Figures METHOD OF IMPROVING STABILITY OF BORON HARDENABILITY EFFECT IN ALLOY STEELS CROSSREFERENCES TO RELATED APPLICATIONS This is a continuation-in-part of our copending application Serial No. 824,749 filed May 8, 1969 now abandoned which was a streamlined continuation of our copending application Serial No. 541,210 filed April 8, 1966, which is now abandoned.

BACKGROUND OF THE INVENTION This invention relates broadly to quench hardenable steels and more particularly to hypoeutectoid boron steels having more stable hardenability than the steels of the prior art.

Boron steels, i.e., steels containing about 0.0004 percent to 0.010 percent boron, are becoming increasingly popular largely because such steels provide improved hardenability at relatively low cost. One problem encountered with boron steels lies in the fact that exposure to high temperatures, such as occurs during heating for hot work, may result in loss of hardenability. The reason for this loss of hardenability has not been clear and no solution for the problem has heretofore been offered.

The principal object of this invention is to provide a boron steel having stabilized hardenability.

Another object of this invention is to extend the use of boron steels.

Still another object of this invention is to eliminate the loss of hardenability encountered when boron steels are exposed for normal periods of time to normal hot working temperatures. I

Provided that the maximum amount of vanadium, columbium and aluminum is kept within the maximum expressed by the above equation, the time during which boron steels may be heated without loss of hardenability can be still further increased by the presence of zirconium and/or titanium. More specifically, these elements should be present in a minimum amount expressed by the equation:

% Zirconium/6.51 Titanium/3.42 Boron Preferably, the class of low alloy hypoeutectoid steels to which this invention relates are those consisting essentially of, by weight percent, carbon between about 0.10 to 0.60 percent, nitrogen up to a maximum of 0.020 percent, additions of at least one element from the group consisting of titanium and zirconium, and based upon a maximum value for X equal to 0.01247, vanadium up to a maximum of 0.045 percent, columbium up to a maximum of 0.083 percent, aluminum up to a maximum of 0.024 percent, with the balance essentially iron.

When the alloy steel is held within these chemistry ranges and correlated to satisfy the conditions stated above, the hardening capacity, or hardenability, may be maintained even after prolonged exposure to high soaking and rolling temperatures on the order of 2200 F. Typically, when the heating is maintained at temperatures between about 32 and 75 hours, a loss of hardenability results. The boron containing alloys, prepared by the method of this invention, are not so affected.

The compositions of seven hypoeutectoid boronnitrogen steels containing various nitride forming elements are shown as per cent by weight in Table 1 below:

TABLE I Effective Heat element C Mn 1 S Si N B Al Cb V Zr Ti 4 B .28 .00 .004 .003 .28 .001 .002 .002 .H E ii To B B-N .30 .04 .003 2 .27 .000 .002 .002 .W .H E To 0 ".41 .20 .07 .003 .003 .20 .011 .002 .058 .E E .Tfi IE1 D Cb .2s .00 .005 .004 .24 .000 .002 .002 .075 .E .W 700 E ..v .28 .08 .005 .004 .24 .000 .002 .002 E .052 m 30 F Zr .30 .07 .004 .004 .24 .007 .002 .002 .T .(i .10 110 G Ti .224 .05 .003 .005 .27 .010 .002 .001 m E E .00

SUMMARY OF THE INVENTION Broadly this invention is based on the discovery that the stability of the hardenability effect of boron in steels which contain nitrogen in any amount over X, where X equals Boron .0004/.77) and are exposed to elevated temperatures is decreased by the presence of aluminum, columbium and vanadium, and is increased by the presence of zirconium and/or titanium as hereinafter more fully explained.

Referring to the drawings, FIGS. l-7 are graphs illustrating the effect of time at a normal hot working temperature on the hardenability of seven hypoeutectoid boron steels.

In practicing this invention the amount of the unstable nitride-forming elements, aluminum, columbium and vanadium, is limited to a maximum amount expressed by the equation:

% Vanadium/3.63 columbium/6.63 +9? Aluminum/1.93 Boron .0004/.77

A lb. vacuum induction furnace heat of each of the above compositions was cast into a 6% inch square ingot which was hot rolled into 1 inch diameter rods and cut into 6 inch lengths. Representative 6 inch lengths of each heatwere held at 2200 F. for various periods of time up to 96 hours. These specimens were machined to k inch diameter by 4%: inches long and subjected to a Jominy-type end quench from 1,500 F. Longitudinal flats were ground 0.05 inches deep along two opposing sides of each specimen to provide surfaces for Rockwell C hardness measurements at l/l6 inch intervals from the quenched end.

FIGS. 1-7 illustrate the effect of time at an elevated temperature on the stability of the hardenability effect of boron of these seven steels. The variation in hardenability was measured by the difference in the Rockwell C hardness at locations one-sixteenth inch and four-sixteenth inch from the quenched end of end quenched hardenability specimens. A single data point is indicated on the curves when the difference in the hardness determinations was the same on opposite sides of the specimens and two data points are indicated when the difference was not identical. it is notable that each of the seven steels eventually shows a difference of approximately twenty one points of Rockwell C hardness due to the eventual loss of the effect of boron on hardenability.

A comparison of the seven graphs appearing on FIGS. 1-7 show that the presence of nitride-forming elements may either accelerate or delay the loss of the effect of boron on hardenability when the steel is exposed to an elevated temperature. FIG. 1 representing heat A shows that in the absence of significant quantities of nitrogen, the effect of boron on hardenability is relatively stable for approximately 36 hours, then deteriorates rapidly and is completely eliminated after approximately 44 hours at 2200 F. FIG. 2 representing heat B shows that the addition of small amounts of nitrogen reduces the period during which the effect of boron on hardenability is stable to approximately 31 hoursv FIGS. 3, 4 and 5, representing heats C, D and B respectively, show that in the presence of nitrogen, the presence of the nitride-forming metals aluminum, columbium and vanadium, reduces the period during which the effect of boron on hardenability is stable to less than four hours at 2200 F. FIGS. 6 and 7 representing heats F and G respectively, show that in the presence ofnitrogen the group of nitride-forming metals consisting of zirconium and titanium prolongs the period during which the effect of boron on hardenability is stable to approximately 68 and 78 hours respectively at 2200" F.

Heat G of Table l is a specific example of a preferred composition of a boron-nitrogen steel of this invention which contains only very small amounts of aluminum, columbium and vanadium and which contains titanium in excess of the amount necessary to combine with the nitrogen.

The carbon, manganese and silicon contents of the quench hardenable steels of this invention may be varied from the limits shown and it is also possible to add additional alloying elements such as nickel, chromium, molybdenum and copper in the amounts which are known to contribute to hardenability without affecting the stability of the hardenability effect of boron. It should also be recognized that thestability of the hardenability effect of boron is indirectly proportional to both time and temperature and the previously mentioned limitations are only intended to be used in an illustrative manner. The limitations of this invention are shown in the following claims:

All references are to weight percentages.

We claim:

1. A method of maintaining the high hardening capacity of a boron containing hypoeutectoid low alloy steel consisting essentially of carbon, boron, nitrogen, zirconium, titanium, Columbium, vanadium, aluminum, and the balance iron, comprising the steps of:

a. preparing a molten alloy, by weight as follows: carbon between about 0.10 to 0.60 percent, boron between about 0.0004 to 0.010 percent, nitrogen up to a maximum of 0.020 percent, vanadium up to a maximum of 0.045 percent, columbium up to a maximum of 0.083 percent, aluminum up to a maximum of 0.024 percent, and the balance essentially n. b. correlating the chemistry within the said limits,

where the minimum nitrogen is greater than X with X boron .0004

where the total of percent vanadium/3.63 and percent columbium/6.63 and percent aluminum/1.93 X

0. adding a quantity of at least one element from the group consisting of titanium and zirconium such that the total percent zirconium/6.5l and percent titanium/3.42 B X,and

. solidifying said molten alloy and processing same to final form by subjecting it to a prolonged heating of from about 32-hours to about hours in the normal hot working temperature range between the A0 and the melting point thereof, without affecting the duration of the hardenability effect due to the boron of the said alloy,

. working to reduce the area of the cross-section while the steel is within the normal hot working temperature range, and

f. quenching the hot worked steel from a normal quenching temperature in the range between the Ac temperature and the Ac temperature 350C. 

